Advances in reciprocating engine technology have resulted in substantially lower heat rates, the fuel to kilowatt efficiency exceeding 35%. As a result, newer higher efficiency engines can only produce low grade heat which in most applications precludes its use as the prime mover in the generation of steam for industrial use. Approximately 40% of the waste heat from a reciprocating engine is contained in the engine exhaust which ranges in temperature between 800 and 900 F, the remaining waste heat expelled to the engine cooling water at a temperature no higher than 240 F. Exhaust temperature from gas turbines is much higher. As a result, most cogeneration systems use lower efficiency prime movers such as gas turbines that have sufficient exhaust volume and temperature to produce higher grade heat needed for the production of steam.
About 40% of the consumption of energy by America industry is devoted to the production of steam in boilers.
Numerous combustion processes incident to power generation, generate effluent gases having an unacceptable NOx content. More specifically, the high temperatures incident to the operation of fuel-driven turbines, internal combustion engines and the like, results in the fixation of some oxides of nitrogen. These compounds are found in the effluent gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO2) and only traces of other oxides. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO2) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO2) in a sample is determined and referred to as “oxides of nitrogen” (NOx).
NOx emissions from stack gases, engine exhausts etc., through atmospheric reactions, produce “smog” that stings eyes and may cause or contribute to acid rain. Other deleterious effects both to health and to structures are believed to be caused directly or indirectly by these NOx emissions. For these reasons, the content of oxides of nitrogen present in gases vented to the atmosphere has been subject to increasingly stringent limits via regulations promulgated by various state and federal agencies.
In recent years a mode of power production known as “cogeneration” has expanded rapidly, due in part to the Public Utility Regulatory Policy Act of 1978 (PURPA). PURPA provided financial incentive to cogenerators that sell excess electrical power and indeed mandated that utilities purchase power from cogenerators. It also allows utilities to own up to 50% of a cogeneration facility and receive the benefits of this status. Cogeneration may be defined as the simultaneous production of both useful thermal energy (usually steam), and electrical energy, from one source of fuel. In a typical system one or more power sources such as gas turbines, may be followed by a waste heat boiler using natural gas as fuel for both the turbines and to heat the exhaust gases from the turbines.
A common problem arising in cogeneration systems is the level of NOx emissions generated with the combined firing cycle. Cogeneration plants using conventional hydrocarbon-fueled power sources and auxiliary fuel fired heat-recovery boilers to produce electricity and steam are therefore being subjected to stringent NOx emission standards requiring levels below the 9 ppmv range.
Higher efficiency reciprocating engines have NOx emissions several orders of magnitude higher than these emission standards, often exceeding 1500 ppmv. Therefore, the use of higher efficiency reciprocating engines are precluded in most states without substantial emission control technology.
To meet the regulations for NOx emissions for boilers, a number of methods of NOx control have previously been employed or proposed. In one approach water or steam are injected into the combustion zone. This lowers the flame temperature and thereby retards the formation of NOx, since the amount of NOx formed generally increases with increasing temperatures. Water or steam injection, however, adversely affects the overall fuel efficiency of the process as energy is absorbed to vaporize the water or heat the injectable steam, which would otherwise go toward heating the power source exhaust and be ultimately converted into usable steam.
A much more common technique for meeting regulations NOx emissions involves the use of recirculated flue gas. Flue gas exiting a boiler is mixed with combustion air which reduces oxygen content in the combustion air flue gas mixture to less than 20.9%. The reduction of available oxygen in the burner combined with the higher mass flow reduces the emissions of NOx emissions. Applicant's U.S. Pat. No. 5,511,971 describes a process for reducing emissions from a boiler by passing a mixture of recirculated flue gas and combustion air through a fan and into a burner while increasing the speed of the fan at increasing firing levels. The process has demonstrated the capability of reducing NOx emissions by greater than 85% regardless of burner design.
Specialized burner designs have been developed which in combination with increasing levels of recirculation can provide NOx emissions to meet the most strict emission limits in California. Unfortunately, the increased levels of recirculated flue gas results in substantial loss of flame stability which limits the range of operation as well as the efficiency of the approach.
It is also known to inject ammonia to selectively reduce NOx. A process involving the injection of ammonia into the products of combustion is shown, for example, in Welty, U.S. Pat. No. 4,164,546. Examples of processes utilizing ammonia injection and a reducing catalyst are disclosed in Sakari et al. U.S. Pat. No. 4,106,286; and Haeflich, U.S. Pat. No. 4,572,110. While selective reduction methods ammonia injection are expensive and somewhat difficult to control, increasingly strict mandates against NOx emissions have made the use of selective catalytic reduction systems the preferred choice in the control of NOx emissions from boilers.
Temperature necessary for the reduction of the oxides of nitrogen must be carefully controlled to yield the required reaction rates, the placement of catalyst being in the flow of exhaust from boilers where temperature is in the range to assure the highest possible reduction rates.
Apparatus modifications have also been widely used or proposed as a solution to the aforementioned NOx emission problem. These include modifications to the burner or firebox to reduce the formation of NOx. Although these methods can reduce the level of NOx, each has its own drawbacks. Combustion equipment modifications can e.g. affect performance and limit the range of operation.
A selective catalytic reduction system is presently considered by some to be the best available control technology for the reduction of NOx from the exhaust gas of a boilers and, as a consequence, is often required equipment. Currently available selective catalytic reduction systems used for the reduction of NOx employ ammonia injection into the exhaust gas stream for reaction with the NOx in the presence of a catalyst to produce nitrogen and water vapor. Such systems typically have an efficiency of 85-90 percent when the exhaust gas stream is at a temperature within a temperature range of approximately 500° F.-700° F. The NOx reduction efficiency of the system is significantly less if the temperature is outside the stated temperature range and the catalyst may be damaged at higher temperatures.
U.S. Pat. No. 4,354,821 is also of interest in disclosing a system for combusting a nitrogen-containing fuel in such a manner as to minimize NOx formation. The fuel to be combusted is directed through a series of combustion zones having beds of catalytic materials. Air is added to each of two upstream zones to provide fuel-rich conditions to thereby minimize formation of NOx precursors. In a final zone also having a bed of catalytic material, excess air is provided to complete combustion of the fuel.
U.S. Pat. No. 4,811,555, discloses a cogeneration system wherein electrical power is generated by a gas turbine. The gaseous effluent from the turbine, together with sufficient additional fuel to produce a fuel-rich, fuel-air mixture is fed to a boiler to generate steam. Air is added to the gaseous effluent from the boiler to form a lean fuel-air mixture, and this mixture is passed over an oxidizing catalyst, with the resultant gas stream then passing to an economizer or low pressure waste heat boiler for substantial recovery of its remaining heat content. The gas, now meeting NOx emission standards, is then vented to atmosphere.
U.S. Pat. No. 4,811,555, a gas turbine constitutes the primary power source which has a substantially higher heat rate than that of the newer reciprocating gas engines. The NOx levels ultimately achieved therein are quite low, i.e. below about 50 ppmv for the final gases provided for venting. Since, however, NOx levels in the turbine exhaust are not extremely high to begin with (i.e. about 150 ppmv), the actual reduction is only moderate. Where an internal combustion engine (such as a reciprocating engine) constitutes the power source, NOx levels in the exhaust are an order of magnitude higher than in a gas turbine—a typical NOx level for such an engine being about 1500 ppmv. In this instance the exhaust stream also carries substantial particulate matter in the form of unburned carbon. It is found that with such a power source, neither the methods taught in U.S. Pat. No. 4,811,555, or those otherwise known in the prior art which preceded U.S. Pat. No. 5,022,226, are adequate or effective to economically and efficiently achieve fully acceptable NOx reduction. The problem thereby presented is particularly acute, in that the convenience, simplicity of operation, and dependability of internal combustion engines, otherwise renders same an ideal instrumentality for use in cogeneration installations, e.g. for shopping centers, industrial plants, educational facilities, medical complexes, and the like.
In U.S. Pat. No. 5,022,226, a cogeneration system is provided wherein fuel and oxygen are provided to an internal combustion engine connected to drive an electric generator, to thereby generate electricity. An exhaust stream is recovered from the engine at a temperature of about 500° F. to 1000° F. which includes from about 6 to 15 percent oxygen. Sufficient fuel is added to the exhaust stream to create a fuel-rich mixture, the quantity of fuel being sufficient to react with the available oxygen and reduce the NOx in the exhaust stream. The fuel-enriched stream is then provided to a thermal reactor means for reacting the fuel, NOx and available oxygen, to provide a heated oxygen-depleted stream. The oxygen-depleted stream is cooled in a heat exchanger. Prior to being passed over a catalyst bed under overall reducing conditions, conversion oxygen is added to the cooled stream. Such oxygen can be provided directly (i.e. as air), but can be provided by bypassing part of the exhaust stream from the engine. The quantity of conversion oxygen is stoichiometrically in excess of the amount of NO.sub. x but less (stoichiometrically) than the amount of combustibles, in consequence of which NO in the stream is oxidized to NO2 at the forward end of the bed, after which the NO2 is reduced in the remainder of the bed by the excess combustibles. Air is added to the resulting stream from the catalytic bed to produce a cooled stream having a stoichiometric excess of oxygen, and the stream is passed over an oxidizing catalyst bed to oxidize remaining excess combustibles. The resultant stream, vastly reduced in NOx content can then be provided for venting. By means of the U.S. Pat. No. 5,022,226 invention, the NOx content can be reduced to less than 25 ppmv—often below 15 ppmv, while CO levels are also brought to well below 50 ppmv.
What is needed is a better cogeneration power plant.